In prior art encapsulated cholesteric liquid crystals (CLCs) are known for use in decorative or security applications, like for example printing inks, colored images, identification cards, bank notes or other documents of value that should be forge-proof. The CLC material is usually a mixture of low molecular weight CLC compounds that is encapsulated or encased in a transparent shell of e.g. gum arabic or gelatine. Often photochromic or thermochromic CLC mixtures are used which change their color or become colorless when being subjected to photoradiation or to a temperature change, respectively.
CLCs are characterized by a helically twisted structure. A layer of a CLC material with planar alignment, i.e. wherein the cholesteric helix axis is oriented perpendicular to the plane of the layer, shows selective reflection of circular polarized light caused by interaction of incident light with the cholesteric helix. The central wavelength of reflection λ depends on the pitch p and the average refractive index n of the CLC material according to the following equationλ=n·p
However, encapsulated CLCs have several disadvantages. For example, the capsules containing the CLCs exhibit only limited long term stability, and can break and release the CLCs. Furthermore, they are sensitive to heat and pressure. Therefore, whereas encapsulated CLCs may be useful for short term applications such as clothing labels etc., they are less suitable for long term security items such as bank notes.
The inventors have found that the above mentioned drawbacks can be avoided by using solid polymer beads comprising a polymerized anisotropic material with helically twisted structure, like for example polymer beads of polymerized liquid crystal material, instead of a low molecular weight LC mixture encapsulated in a polymer shell. Solid LC polymer beads according to the present invention have better chemical and heat resistance and are therefore more suitable for both short and long term applications than encapsulated LCs.
JP 02-281045 discloses liquid crystalline high polymer spherical particles of a thermotropic polymer, like e.g. a polyester, which can be used as particulated moulding material, sinter-molding material, filler for thermoplastic and thermosetting plastic material and filler of heat-resistant paint, enamel and additives. However, JP 02-281045 does not disclose LC polymer beads with a helically twisted structure.